Crash prevention devices for machine tools are known in the field of workpiece machining with machine tools like milling, turning or electrical discharge machining (EDM). The tools used in machine tools—typically milling, drilling or swivel heads—are normally interchangeable and depend on the machining process to be performed. Quite often the machining of one workpiece requests the use of several different tools. The tool change can be conducted manually by an operator, semi-automatically or fully automated (e.g. in computerized machining centers). Such tool changes request in the outmost cases a non-machining movement of the employed tool respectively of the machining head to which the tool to be changed is attached, away from workpiece or towards the workpiece for continuing the machining process. During these and other non-machining movements of the employed tool (e.g. a new part setup process) as during normal machining movements, an inherent risk of collision respectively crash of the machining head or of the thereon clamped tool with the workpiece or with other machine parts always exists. Such a collision is highly undesired because it can damage a workpiece, the tool or, worse, the machine tool. Since in such situations the tools are always clamped in the chuck of the machining head, a collision can imply severe damages on the support or mounting of the machine head or on other machine elements (e.g. motor spindle, tool or workpiece chuck, machine table etc.), causing therewith a very expensive breakdown of the entire machine tool for a longer time and expensive reparation costs. Particularly unpleasant are smaller crashes which occur undetected causing the imprecise (mass) production of workpieces respectively subsequent massive discards.
Avoiding these types of collisions and crashes has therefore always been an important part of operating a machine tool. In machine tools operated manually, in numerically controlled machine tools or in computerized machining cells, the clamped workpiece and its exact position is first measured. This is done to ensure that for starting the subsequent machining process, the tool is not crashing in the workpiece, but first precisely positioned by a non-machining movement of the tool respectively machining head just next to the workpiece. It is obvious that a visual observation of the non-machining movement of the tool by the operator helps to prevent undesired crashes (although human failure can never be excluded completely). In semi and particularly in fully automated machine tools a constant visual observation is of course neither desired nor possible considering the travel speed of modern machining heads. A necessity for reliable crash prevention systems is consequently given today in this technical field.
In the past years efforts were made in this direction. The document U.S. Pat. No. 6,481,939 B1 discloses for instance a tool tip conductivity sensor. The device detects when a tool tip of a machine contacts a workpiece. The apparatus is intended for an accurate determination of the first contact of the tool with the workpiece to ensure the correct subsequent machining of the workpiece, respectively the further machining operation (in the given example for the subsequent drilling of a workpiece with the tool).
Another publication, the WO 2006/128892 A1, illustrates a device and method for detecting tool breakages—here drilling heads—during a machining operation by using electrical conductivity. The disclosed system measures an electrical variable, which is compared from time to time with threshold values in order to detect deviations deriving from possible tool malfunctions and breakages. The machine can be stopped automatically once such a deviating signal is detected.
The document EP 2 165 803 B1 discloses a similar but more sophisticated control system for controlling the state of a tool during machining operations. The device can monitor high-speed micromachining operations, offering the possibility to control micro tools used in such applications. The system uses alternating current as a signal for its measuring purpose. The signal is produced by a generator and measured by a voltage measuring device. The (high) frequency of the alternating current—in the range of 1 to 60 MHz—is chosen in dependence of the monitored electrical circuit (mainly comprising machine head, tool and machine table). The contact or non-contact of the used micro tool with the workpiece causes a difference in the impedance of that electrical circuit, which is measured by the voltage measuring device. Alternations of that circuit, such as tool-breakages, cause variations of the voltage in the impedance. Therewith it is possible to control the micro tool and monitor its operational condition, such as the normal machining operation, the breaking of the used work tool or even occurred machine collisions. In fact, the system offers the possibility to foresee the stop of the movement of the machine once a collision is detected.
The disadvantage of the control system according to EP 2 165 803 B1—as of all other foregoing cited technical solutions—is that a collision is just detected once it has already occurred. While in micro machining operations—with minimal feed rates respectively tool movements—a system according to the EP 2 165 803 B1 might still be sufficient to prevent severe damages, such a system is not adequate for ordinary or high speed milling machines with feed rates and tool movements in complete other magnitudes.
Further, it is known on the market to utilize crash avoidance systems. These systems function on a model-basis, assuming the position of workpiece or other potential colliding elements according to data feed to the system. Does the tool/machine head for example approach the assumed position of the workpiece, the system slows down the movements to avoid an eventual collision with the workpiece. The limit of such an approach is that only a sub set of the colliding elements can be taken into account by the model or the model is inaccurate (i.e. missing or approximated elements). The company Heidenhain sells for instance such a system, known on the market as “DCM”.
Further, mechanical crash mitigation systems are also known and disclosed for instance in the documents EP 1 398 110 A1 and DE 10 2007 032 498 A1. Such systems prevent bigger and severe damages, but cannot contribute to avoid all damages by example at all possible speeds or with all possible tool geometries and therefore require the machine tool to be used at a reduced speed.
In ordinary and particularly in high speed milling machines a collision should not only be mitigated, but imminent collisions should preferably be detected before they actually occur to permit therewith the stoppage of the movement of a tool/machining head right in time, avoiding that a tool crashes in the workpiece or in other machine elements at all. Only in that way it is possible to avoid any damage.
An approach in this direction is given by the system disclosed in the document EP 2 402 11 B1 of the company Ott-Jakob Spanntechnik GmbH. This document discloses a plurality of distance sensor which are allocated at the end of the motor spindle circumferentially around the axis of the spindle respectively around the mounted tool. These sensors have an overlapping detection field and work with radar or ultrasound sensing means. Such a system is able to prevent a crash, however the requested equipment for the detection system is rather sophisticated and complex. Further it requests quite expensive sensor means and today these systems are still not accurate enough for industrial application.